This specifically discusses pain while pregnant and focuses in the pelvic girdle area:Pregnancy related Pelvic Girdle Pain (PGP) causes pain, instability and limitation of mobility and functioning in any of the three pelvic joints. PGP has a long history of recognition, mentioned by Hippocrates[1] and later described in medical literature by Snelling.[2]

The affection appears to consist of relaxation of the pelvic articulations, becoming apparent suddenly after parturition or gradually during pregnancy and permitting a degree of mobility of the pelvic bones which effectively hinders locomotion and gives rise to the most peculiar and alarming sensations.—Snelling (1870), [2]

Here is the anatomy of the pelvis, according to the Wikipedia.org article:

The pubic symphysis has a fibrocartilage joint which may contain a fluid filled cavity and is avascular; it is supported by the superior and arcuate ligaments. The sacroiliac joints are synovial, but their movement is restricted throughout life and they are progressively obliterated by adhesions. The nature of the bony pelvic ring with its three joints determines that no one joint can move independently of the other two.[3]